An optical fiber (i.e., a glass fiber typically surrounded by one or more coating layers) conventionally includes an optical fiber core, which transmits and/or amplifies an optical signal, and an optical cladding, which confines the optical signal within the core. Accordingly, the refractive index of the core nc is typically greater than the refractive index of the optical cladding ng (i.e., nc>ng).
For optical fibers, the refractive index profile is generally classified according to the graphical appearance of the function that associates the refractive index with the radius of the optical fiber. Conventionally, the distance r to the center of the optical fiber is shown on the x-axis, and the difference between the refractive index (at radius r) and the refractive index of the optical fiber's outer cladding (e.g., an outer optical cladding) is shown on the y-axis. The refractive index profile is referred to as a “step” profile, “trapezoidal” profile, “alpha” profile, or “triangular” profile for graphs having the respective shapes of a step, a trapezoid, an alpha, or a triangle. These curves are generally representative of the optical fiber's theoretical or set profile. Constraints in the manufacture of the optical fiber, however, may result in a slightly different actual profile.
Generally speaking, two main categories of optical fibers exist: multimode fibers and single-mode fibers. In a multimode optical fiber, for a given wavelength, several optical modes are propagated simultaneously along the optical fiber. In a single-mode optical fiber, the signal propagates in a fundamental LP01 mode that is guided in the fiber core, while the higher order modes (e.g., the LP11 mode) are strongly attenuated.
Conventionally, so-called “standard” single mode fibers (SSMFs) are used for land-based transmission systems. To facilitate compatibility between optical systems from different manufacturers, the International Telecommunication Union (ITU) defined a standard reference ITU-T G.652 with which a standard optical transmission fiber (i.e., a standard single-mode fiber or SSMF) should comply. The ITU-T G.652 recommendations and each of its attributes (i.e., A, B, C, and D) are hereby incorporated by reference.
Typically, an SSMF complies with specific telecommunications standards, such as the ITU-T G.652 recommendations. Conventionally, an SSMF exhibits the following properties: (i) at a wavelength of 1550 nanometers (nm), attenuation of about 0.190 decibels per kilometer (dB/km); (ii) at a wavelength of 1550 nanometers, an effective area of about 80 square microns (μm2); (iii) a 22-meter cable cutoff wavelength of less than 1260 nanometers; (iv) a positive chromatic dispersion of about 17 picoseconds per nanometer kilometer (ps/(nm·km)); and (v) at a wavelength of 1550 nanometers, a positive dispersion slope of 0.058 picoseconds per nanometer square kilometer (ps/(nm2·km)).
For wavelength division multiplexing (WDM) applications, single-mode non-zero dispersion shifted fibers (NZDSFs) are also used. An NZDSF exhibits a chromatic dispersion at a wavelength of 1550 nanometers that is less than the chromatic dispersion of an SSMF. A dispersion shifted fiber presenting non-zero chromatic dispersion that is positive for the wavelength at which it is used (e.g., about 1550 nanometers) is commonly referred to as an NZDSF+. At a wavelength of 1550 nanometers, an NZDSF+ typically presents a chromatic dispersion of between about 3 ps/(nm·km) and 14 ps/(nm·km), and a chromatic dispersion slope of less than 0.1 ps/(nm2·km). An NZDSF+ typically complies with specific telecommunications standards, such as the ITU-T G.655 and ITU-T G.656 recommendations. The ITU-T G.655 and ITU-T G.656 recommendations are hereby incorporated by reference.
Conventionally, an NZDSF has a triple-clad structure (i.e., a triple-clad NZDSF). An exemplary NZDSF includes: (i) a central core having a refractive index difference with respect to an outer cladding (e.g., and outer optical cladding); (ii) a first inner cladding (e.g., an intermediate cladding) having a refractive index difference with respect to the outer cladding; and (iii) a second inner cladding (e.g., a ring) having a positive refractive index difference with respect to the outer cladding. The refractive indices in the central core, the intermediate cladding, and the ring are substantially constant over their entire widths. Conventional NZDSFs are commercially available, such as eLEAF® fiber, TrueWaveRS® fiber, or Draka Communications' TeraLight® fiber.
An NZDSF may have a coaxial refractive index profile (i.e., a coaxial NZDSF). The central core of an NZDSF having a coaxial refractive index profile includes two zones. The first zone is located in the center of the central core and has a refractive index difference with respect to the outer cladding that is less than that of the second zone. The second zone has a positive refractive index difference with respect to the outer cladding. The first zone's refractive index difference with respect to the outer cladding may be positive, negative or even zero.
An NZDSF may also include: a central core; an inner cladding; and a buried trench (i.e., a cladding layer having a negative refractive index difference with respect to the outer cladding). Typically, this kind of profile is simpler to fabricate. Additionally, for approximately identical optical characteristics, this kind of NZDSF's central core has a refractive index difference that is less than a triple-clad NZDSF's central-core refractive index difference. Consequently, less central core doping is required to achieve this kind of NZDSF, which in turn reduces signal attenuation, particularly attenuation losses caused by Rayleigh diffusion.
In use, optical fibers may be subjected to bends that attenuate the signals conveyed by the optical fiber. Minimizing an optical fiber's bend loss typically improves the quality of the signal conveyed.
Optical fibers may be designed to ensure that the transmitted signal's higher order modes are attenuated sufficiently (i.e., with respect to the fundamental mode) to obtain single-mode transmission. An optical fiber's ability to attenuate higher order modes varies depending on the wavelength of the transmitted signal. The so-called “cutoff wavelength” is the wavelength from which the higher order modes are attenuated sufficiently to ensure that transmission is single-mode. The cutoff wavelength characterizes a signal's single-mode transmission as a function of its wavelength.
Generally speaking, improving certain optical characteristics can have a detrimental effect on other optical characteristics, which can reduce an optical fiber's compatibility with other optical fibers. Thus, it is generally desirable to improve certain optical characteristics while maintaining suitable compatibility between optical fibers.
The article “New Medium-Dispersion Fiber with Large Effective Area and Low Dispersion Slope” by S. Matsuo, et al., published in Optical Fiber Communication Conference and Exhibit 2002, OFC 2002, Vol., Issue, Mar. 17-22, 2002, pp. 329-330, which is hereby incorporated by reference in its entirety, describes a coaxial NZDSF. The disclosed optical fiber's cutoff wavelength is not suitable for all commercial applications. Moreover, the optical fiber's central core includes a zone having a refractive index difference greater than 13×10−3. Such a high refractive index difference can give rise to strong attenuation at a wavelength of 1550 nanometers, such as attenuation greater than 0.21 dB/km (e.g., 0.22 dB/km or more).
European Patent No. 0,992,817 and its counterpart U.S. Pat. No. 6,459,839, each of which is hereby incorporated by reference in its entirety, describe a triple-clad NZDSF that possesses low bending loss. Nevertheless, the disclosed optical fiber's cutoff wavelength is not suitable for all commercial applications. Moreover, for comparable optical characteristics, the optical fiber's central core has a refractive index difference of about 13.7×10−3, which is greater than in an optical fiber that includes a buried trench. At a wavelength of 1550 nanometers, the disclosed optical fiber, therefore, exhibits attenuation that is greater than 0.20 dB/km, or even greater than 0.21 dB/km. These attenuation values are greater than in an optical fiber that includes a buried trench. Additionally, the disclosed triple-clad NZDSF is more difficult to manufacture than an optical fiber that includes a buried trench, because the parameters of the triple-clad NZDSF's ring are more sensitive and require fabrication tolerances that are smaller than those for an effective buried trench.
European Patent No. 1,477,831 and its counterpart U.S. Pat. No. 6,904,218, each of which is hereby incorporated by reference in its entirety, describe the use of a buried trench to improve the optical characteristics of an SSMF. Similarly, European Patent No. 1,978,383 and U.S. Patent Publication No. 2005/0244120, each of which is hereby incorporated by reference in its entirety, describe the use of a buried trench to improve the optical characteristics of an SSMF. Nevertheless, these documents fail to disclose an NZDSF with improved bending losses and a suitable cutoff wavelength.
U.S. Pat. No. 4,852,968, which is hereby incorporated by reference in its entirety, describes the use of a buried trench placed close to the central core to decrease the values of chromatic dispersion and chromatic dispersion slope. Nevertheless, the disclosed optical fiber has a ratio of inside trench radius to central core radius that is between about 1.5 and 3.5, which can give rise to (i) large bending loss values for radii of 30 millimeters (mm), and (ii) at a wavelength of 1550 nanometers, an effective area of less than 55 μm2.
International Patent Application Publication No. WO2008/106033 and its counterpart U.S. Pat. No. 7,603,015, each of which is hereby incorporated by reference in its entirety, present NZDSFs that include a buried trench. Nevertheless, the central core has a refractive index difference that is too small and a radius that is too great. The characteristics of the central core give rise to excessive bending losses at large radii of curvature (e.g., greater than 25 millimeters). The disclosed NZDSFs possess a chromatic dispersion slope that is greater than 0.060 ps/(nm2·km) at a wavelength of 1550 nanometers. For example, the present inventors have calculated that, at the wavelength of 1625 nanometers and a radius of curvature of 30 millimeters, the exemplary NZDSF exhibits bending losses of greater than 10 decibels per 100 turns (dB/100 turns).
Therefore, a need exists for an NZDSF that exhibits low bending losses for large radii of curvature and improved single-mode transmission, while maintaining suitable compatibility with other optical fibers.